Molal Volumes of the Isotopic Homologs of Ethylene

The molal volumes of ethylene (C2H4), trans-dideuteroethylene (C2H2D2), and tetradeuteroethylene (C2D4) have been measured over the temperature range 105–175°K with a precision 10−2% (± 0.005cm3). The order of the molal volumes is VC2H4 > VC2H2D2 > VC2D4. The difference in molal volume between ethylene and each of its deutero derivatives is approximately 0.05 cm3 mol−1 per deuterium atom and decreases with temperature. The difference in molal volumes is compared with two limiting models. The “size effect” model, which is a consequence of the difference in lengths of the C–H and C–D bonds leads to the correct magnitude of the difference in molal volume, but predicts δV / V to be temperature independent for ethylene, contrary to the experimental findings. Calculations based on an anharmonic intermolecular potential show that the hindered rotation in liquid ethylene makes a significant contribution to the molal volume difference. Finally a correlation is made between the observed molal volume differences and the measured vapor-pressure differences of the deuteroethylenes.